Differential expression of CYB5A in Chinese and European pig breeds due to genetic variations in the promoter region

Cytochrome b5 (CYB5A) is an important electron transfer protein with homologues in a number of different organisms. In pigs, CYB5A is related to boar taint because of its role in androstenone biosynthesis. To determine the variety of CYB5A expression in pig breeds, genetic variations in the porcine CYB5A promoter region in both Chinese and European pig breeds were examined. Three single nucleotide polymorphisms (NC_010443.4:g.165901487delG, g.165901767T>C and g.165902078C>T) were identified in the porcine CYB5A promoter region. These SNPs occurred in different frequencies in Chinese and European pigs. Chinese pigs were primarily haplotype B (denoted as delG-C-T: the position of nt 165901487 of the CYB5 gene is a G deletion, nt 165901767 is C and nt 165902078 is T), except for Licha black pigs, which were primarily haplotype A (denoted as G-T-C: nt 165901487 is G, nt 165901767 is T and nt 165902078 is C), similar to European pigs. Quantitative PCR data from liver tissues demonstrated that haplotype B individuals had higher CYB5A expression than did those with haplotype A. This was confirmed by in vitro cell transfection assays, in which haplotype B individuals had higher reporter activity than did those with haplotype A. In silico analysis predicted that Myc-associated zinc-finger protein (MAZ) is a potential transcription factor at position 165901767. Electrophoretic mobility shift assays showed this polymorphism affects the stable binding of transcription factors to the CYB5A promoter, which in turn affects the expression levels of this gene. Therefore, this variation of the porcine CYB5A promoter region may explain the differences in androstenone accumulation between Chinese and European pig breeds and may also prove useful as a genetic marker to distinguish the origin of different pig breeds.